Fuel cells have been proposed as a power source for electric vehicles, stationary power supplies and other applications. One known fuel cell is the PEM (i.e., Proton Exchange Membrane) fuel cell that includes a so-called MEA (“membrane-electrode-assembly”) comprising a thin, solid polymer membrane-electrolyte having an anode on one face and a cathode on the opposite face. The MEA is sandwiched between a pair of electrically conductive contact elements which serve as current collectors for the anode and cathode, which may contain appropriate channels and openings therein for distributing the fuel cell's gaseous reactants (i.e., H2 and O2/air) over the surfaces of the respective anode and cathode.
PEM fuel cells comprise a plurality of the MEAs stacked together in electrical series while being separated one from the next by an impermeable, electrically conductive contact element known as a bipolar plate or current collector. In some types of fuel cells each bipolar plate is comprised of two separate plates that are attached together with a fluid passageway therebetween through which a coolant fluid flows to remove heat from both sides of the MEAs. In other types of fuel cells the bipolar plates include both single plates and attached together plates which are arranged in a repeating pattern with at least one surface of each MEA being cooled by a coolant fluid flowing through the two plate bipolar plates.
The fuel cell stacks are typically part of a fuel cell system that is operated to meet a power demand placed upon the fuel cell system. The power demand placed upon the fuel cell system, however, can vary over time for a variety of reasons. For example, when the fuel cell system is on a mobile platform, such as a vehicle, the power demand placed upon the fuel cell system will vary with the desired acceleration and deceleration of the mobile platform. When the fuel cell system is used in a stationary application, the power demand placed upon the fuel cell system will also vary. For example, when a furnace, refrigerator, electric dryer, etc. are switched on and off, the power demand placed upon the fuel cell system will change. In response to the change in the power demand placed upon the fuel cell system, the quantity of anode reactant supplied to the fuel cell stack is typically adjusted to meet the power output demanded of the fuel cell stack. When the power demand placed upon the fuel cell system is decreased (downward transient) the quantity of anode reactant supplied to the fuel cell stack is reduced so that the power output of the fuel cell stack decreases to approximately match the power demand placed on the fuel cell system.
During fast downward transients, however, the current flow through the fuel cell stack decreases much more rapidly than the fuel cell stack can compensate for. For example, if the system is run at a high load at a steady state, the anode and cathode pressures are high, and the operating temperature is at its upper operating range. During a downward transient, the load placed upon the fuel cell system and fuel cell stack goes down very quickly. Ideally, the cathode flow and anode flow should ramp down as quickly, as well as system pressure and coolant temperature. These different parameters, however, have much different physical time constraints. Thus, the load can drop almost instantaneously while the anode and cathode flows may take several seconds and the coolant may take significantly longer to drop to the new operating condition.
This delay in adjusting these different parameters to the new decreased power demand placed upon the fuel cell stack can potentially cause damage to the individual fuel cells and/or fuel cell stack potentially resulting in instantaneous failure of the fuel cell stack and/or a decreased longevity of the fuel cell stack. Thus, it would be advantageous to operate the fuel cell system in a manner that prevents or minimizes the potential for damage to the fuel cells and/or fuel cell stack during downward transients in the power demand placed upon the fuel cell system.